1. Field of the Invention
The present invention relates to a position sensitive detector (PSD) for detecting positioning of an incident beam of light (light input) employing a photoelectric conversion device, and more particularly, to a method and apparatus for detecting position/variance of input light suitable for use, for example, in the detection of surface flaws, surface roughness, or surface shape of metallic materials and the like utilizing a beam of light or detection of defocusing produced in a measuring instrument which uses a beam of light.
2. Description of the Prior Art
FIG. 18 is a side view of a prior art position sensitive detector, for example, as described in a paper entitled "An Application of PSD to Measurement of Position", by Kazuo Kurasawa, pp. 730-737, Precision Instrument, Vol. 51, No. 4, 1985.
In this prior art position sensitive detector, a flat silicon substrate is provided as shown in FIG. 18 with a first resistive layer 51 formed of a p-type semiconductor (photoelectric conversion material) to receive a light input and a second resistive layer 52 formed of an n-type semiconductor (photoelectric conversion material) connected with the first resistive layer 51 through a depletion layer 53, in which the first resistive layer 51 is provided with uniform resistivity throughout its surface.
In order to detect the one-dimensional position and light energy of a beam of light incident on the first resistive layer 51, there are provided electrodes 54, 55 attached to both ends of the first resistive layer 51.
Further, in the center of the second resistive layer 52, there is connected a bias electrode 56.
With such a position sensitive detector, when a beam of light impinges on the first resistive layer 51 as shown by the arrow a in FIG. 18, its one-dimensional position and light energy is obtained in the following manner. In this case, the coordinate system is set up such that the middle point of the electrodes 54, 55 is taken as the origin, the coordinate of the position of incidence with respect to the origin is represented by x, and the positive direction is taken to the right in FIG. 18.
When the beam of light impinges on the first resistive layer 51 as indicated by the arrow a, there are generated electrons and positive holes proportional to the light energy at the position of incidence. The generated positive holes flow through the first resistive layer 51 and are detected as current values I.sub.10, I.sub.20 from the electrodes 54, 55, respectively.
The generated electron flow through the depletion layer 53 into the second resistive layer 52 and are taken out from the bias electrode 56.
At this time, the flow of the positive holes, i.e., the photoelectric current, is distributed in inverse proportion to the distances from the position of incidence to the electrodes 54, 55 on account of the uniform resistivity of the first resistive layer 51. Accordingly, representing the total length of the first resistive layer 51 by 2L, we obtain EQU I.sub.10 /(L-x)=I.sub.20 /(L +x)=k (1)
where k is a quantity depending on the light energy, not on the position of incidence x.
Then, if (I.sub.10 -I.sub.20)/(I.sub.10 +I.sub.20) in connection with the measured values I.sub.10, I.sub.20 is obtained from equation (1), it is expressed as ##EQU1## and therefore, the position of incidence x can be measured regardless of the light energy according to EQU x=L.multidot.(I.sub.10 -I.sub.20)/(I.sub.10 +I.sub.20) (3)
And, since (I.sub.10 +I.sub.20) is equal to 2k.multidot.L, the quantity k depending on the light energy is detected regardless of the position of incidence k according to EQU k=(I.sub.10 +I.sub.20)/(2L) (4)
As described above, the one-dimensional position of the beam of light incident on the first resistive layer 51 and the quantity k depending on the light energy can be obtained from the current values I.sub.10, I.sub.20 measured with the position sensitive detector according to equations (3), (4). Incidentally, the two-dimensional position of the beam of light can also be measured on the same principle as utilized in the above described measurement of the one-dimensional position.
In such case, a position sensitive detector as shown in FIG. 19 or FIG. 21 is used.
The position sensitive detector as shown in FIG. 19 is of a surface division type, in which two pairs of electrodes 57, 58 and electrodes 54, 55 are disposed at both ends of a first resistive layer 51 in the directions at right angles to each other. In such a position sensitive detector, while the .times. coordinate is obtained based on equation (3) from current values I.sub.10, I.sub.20 detected from the electrodes 54, 55 respectively, the y coordinate is obtained from current values I.sub.30, I.sub.40 detected from the electrodes 57, 58, respectively. Incidentally, FIG. 20 is an equivalent circuit diagram of the position sensitive detector as shown in FIG. 19, and referring to the FIG., P denotes a power source, D denotes an ideal diode, Cj denotes junction capacitance, R.sub.sh denotes a resistor in parallel, and R.sub.p denotes the positioning resistor.
The position sensitive detector as shown in FIG. 21 is of a double-side division type, in which the electrodes 57, 58 are disposed at both ends of the second resistive layer 52 at right angles to the direction between the electrodes 54, 55. In this case, the second resistive layer 52 is also provided with uniform resistivity throughout its surface.
Also with such a position sensitive detector, the x coordinate is obtained from the current values I.sub.10, I.sub.20 detected from the electrodes 54, 55 while the y coordinate is obtained from the current values I.sub.30, I.sub.40 detected from the electrodes 57, 58. Here, FIG. 22 is an equivalent circuit diagram of the position sensitive detector as shown in FIG. 21.
Besides the position sensitive detector using the photoelectric conversion device as described above, there are apparatuses for detecting the position of light using an imaging device (image pick-up tube, solid state imaging element such as CCD camera) or photodiodes.
In the apparatus using an imaging device, an image of a beam of light is picked up by the imaging device and the image is converted into a digital image by analog-to-digital conversion, and the digital image is then processed by signal processing circuits as shown in FIG. 23 and FIG. 25 whereby the position of the beam of light and its spread (variance) are detected, respectively.
The signal processing circuit for detecting the position of a beam of light is, as shown in FIG. 23, made up of a timing pulse generator 60 for outputting an operation instructing signal, a comparator 61 in response to a scan signal of an image from an imaging device 59 for detecting that the signal has exceeded a predetermined threshold value, a pulse generator 62 for generating pulses over the period from the time point when the scanning is started to the time point when the detection signal from the comparator 61 is output, and a counter 63 for counting the number of pulses output from the pulse generator 62 to detect and output the count value as the position of the beam of light. Therefore, in detecting the position of the beam of light, as shown in FIG. 24, when at first the timing pulse generator 60 has output the operation instructing signal, the imaging device 59 starts scanning and simultaneously the pulse generator 62 starts to output the pulse. Then, upon the detection by the comparator 61 of the position of a light signal P.sub.1 exceeding the predetermined threshold value in the scan signal from the imaging device 59, the pulse generator 62 stops its operation. In the meantime, the number of pulses from the pulse generator 62 are counted by the counter 63, and the count value is provided as the position of the beam of light.
The signal processing circuit for detecting the spread (variance) of the beam of light is, as shown in FIG. 25, made up of a comparator 61 in response to a scan signal of an image from an imaging device 59 for detecting that the signal has exceeded a predetermined threshold value, a pulse generator 64, an AND gate 65 performing a logical multiply operation on the pulse signal from the pulse generator 64 and the detection signal from the comparator 61, and a counter 66 for counting the number of pulses passed through the AND gate 64 to detect and output the count value as the spread (variance) of the beam of light. Accordingly, in detecting the spread (variance) of a beam of light, as shown in FIG. 26, the comparator 61 detects the period during which there exists a light signal P.sub.2 exceeding a predetermined threshold value in the scan signal from the imaging device 59, and since the pulses from the pulse generator 64 have been passed through the AND gate 65 during that period, the number of the passed pulses is counted by the counter 63 and this count value is provided as the spread (variance) of the beam of light.
In the apparatus using photodiodes or the like, a plurality of such photodiodes or phototransistors, not shown, are disposed in a line, and it is detected on which photodiode or the like a beam of light impinges, whereby the position and spread (variance) of the beam of light are detected.
Now, the incident beam of light is not a ray but has some spread. The degree of spread, when the beam of light is a reflected beam of light from a surface as an object of inspection, bears information on flaws on or roughness of the surface or shape of it, or in the case of a measuring instrument, the degree of spread bears information about defocusing produced in the instrument. However, with position sensitive detectors using a photoelectric conversion device of the prior art, one can only obtain the position (or, to be more exact, the center-of-gravity position) of the beam of light.
Therefore, in order to detect the spread of a beam of light under existing conditions, it is required to take an image of the beam with an imaging device (such as an image pick-up tube, CCD camera) and subject it to analog-to digital conversion, and to calculate, from the obtained digital image, the spread of the beam of light by means of the signal processing circuit as described above (FIG. 25). Thus, there have been such problems that the apparatus becomes considerably large and quite expensive and laborious steps of procedure must be taken.
With the apparatus employing photodiodes or the like, high resolution is not achievable because a large number of such elements cannot be used, and therefore, such an apparatus has not been useful when high accuracy is desired.